1. Field of the Invention
The present invention relates to a disc rotor for a disc brake and, more particularly, to a disc rotor suitable for a disc brake apparatus for a vehicle.
2. Description of the Related Art
Japanese Laid-Open Utility Model Application No. 59-63238 discloses a disc brake apparatus. The disc brake apparatus disclosed in this patent document comprises a disc rotor rotating with a wheel, a pair of brake pads provided on opposite sides of the rotor, and a caliper supporting the brake pads. The disc rotor includes a hat portion which is fixed to an axle hub connected to a vehicle body via a bearing. Additionally, the disc rotor includes an annular sliding portion on the outer circumference of the hat portion. When a depression force is applied to a brake pedal, the caliper moves the brake pads toward the disc rotor in response to the depression force. As a result, the brake pads are pressed against a slide portion of the disc rotor in response to the depression force.
When the brake pads are pressed against the rotor, a frictional force is generated between the disc rotor and the brake pads. The frictional force acts as a force to restrict the rotation of the disc rotor, that is, a brake force for restricting the rotation of the wheel. Thus, the disc brake apparatus generates the brake force in response to the depression force applied to the brake pedal.
When a brake force is generated by the brake pads being pressed against the disc rotor, vibration is generated in the disc brake due to the sliding of the brake pads on the disc rotor. In such a case, a compressional wave which transmits in a circumferential direction in an annular sliding portion of the disc rotor is generated. Hereinafter, this vibration is referred to as in-plane vibration. Additionally, axial direction vibration is also generated which periodically displaces each portion of the disc rotor in the axial direction. When the frequency of the vibrations matches the natural frequency of the brake system, a so-called brake noise is generated.
If the vibration generated in the disc rotor can be efficiently attenuated, a disc brake having less brake noise can be achieved. Since the in-plane vibration of the disc rotor transmits to the sliding plane of the disc rotor, the in-plane vibration hardly transmits to the caliper. On the other hand, the axial direction vibration is easily transmitted to the caliper since the direction of transmission of the axial direction vibration matches the direction of movement of the caliper.
The vibration generated in the rotor is further attenuated as the vibration is transmitted to a heavier material such as the caliper. Accordingly, as far as reduction of the brake noise is concerned, it is preferred that the vibration, which is generated in the rotor due to the brake pads being pressed against the disc rotor, has a large component in the axial direction. However, in the conventional disc rotor, no measures have been taken to generate the vibration having a large component in the axial direction.